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Assuming it's turning at the same speed as when driven by the engine, it'll use the same amount of fuel regardless of hooking it up to a wheel or a turbine (which won't work - it would require a really big swept area). There's no free lunch.
The wheel-driven idea would cause it to be under-driven at low speeds, so battery voltage may drop and you may save fuel at those times, but the load on the alternator will be higher as the RPM increases and it puts the lost energy back into the battery, so fuel consumption will go up. I can't see any real savings from that approach.
Also worth emphasizing again: discharging a starting battery will dramatically shorten its life. Going alternator-less may be an effective efficiency mod, but it's definitely not a money saving mod unless you have a source of free or cheap batteries.
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